Semiconductor light sources for photonic quantum computing

The isolation of qubits from decoherence is crucial to the prospect of building revolutionary quantum devices. This work is devoted to an optical study of the decoherence on spin qubits in self-assembled quantum dots. This thesis contributes towards a complete understanding of quantum decoherence, of which highlighted discoveries include bypassing the spectral diffusion in neutral quantum dot emission lines; observing for the first time the self-polarization phenomenon of nuclear spins, via the resonance-locking effect on a negatively charged quantum dot; and revealing the limiting factors on hole spin dephasing, by measuring polarization correlations on a positively charged quantum dot. Three studies were conducted using two different spectroscopy techniques. For the first study, the spectral diffusion of emission line due to random electrostatic fluctuations was revealed, by scanning a neutral quantum dot transition across the laser resonance. Exciting the quantum dot resonantly bypassed this problem, paving the way for an on-demand antibunched source that generates narrow-band photons. For the second study, evidences supporting the spontaneous self-polarization of nuclear spins were observed for the first time, since it was predicted nearly four decades ago by M. Dyankonov and V.I. Perel. The self-polarization phenomenon is a remarkable demonstration of dynamic nuclear spin polarization since it manifests without the ground state electron being spin-polarized. In the last study, factors limiting the hole spin lifetime was inferred from measuring polarization correlation of successively emitted photons from a positively charged quantum dot. Evidences support a strong dependence on the carrier repopulation rate and the single electron spin dephasing in the upper state, due to the Overhauser field. In combination with the observation of spontaneous nuclear polarization, this opens the possibility of an electron spin sensor, which can indirectly probe the nuclear field.Open Acces

Increasing the extraction efficiency of quantum light from 2D materials

Direct bandgap 2D semiconductor materials such as monolayers of transition metal dichalcogenides (TMDCs), show great promise in optoelectronic devices enabling exciting new technologies such as ultra-thin quantum light LED’s [1]. These structures can have incredible advantages, enabling almost seamless integration into conventional silicon structures. However, extracting light out of these structures can be a challenge, often requiring costly and time consuming processing e.g. engineered waveguides or cavities [2]. Furthermore none of these methods allow you to observe the light directly, therefore are unhelpful in certain applications, such as an optical version of a quantum unique device [3]. We have previously demonstrated that epoxy based solid immersion lenses can be used to increase light out of semiconductor nanostructures. We furthered this idea to see if they could be used to increase the light out of monolayer TMDC materials; and investigate how the epoxy-2D material interface affects the emission. Our studies revealed that a SIL can greatly enhance the photoluminescence of WSe2 by up to 6x (more than theory predicts for a SIL of this shape), without effecting the wavelength (figure 1). However we also found that the epoxy appears to reduce the emission of the MoS2, suggesting that there could be doping effects due to the epoxy. Overall this method shows great promise as a cheap, and scalable method for enhancing the efficiency of low intensity WSe2 based devices

Photonic crystals for enhanced light extraction from 2D materials

In recent years, a range of two-dimensional (2D) transition metal dichalcogenides (TMDs) have been studied, and remarkable optical and electronic characteristics have been demonstrated. Furthermore, the weak interlayer Van der Waals interaction allows TMDs to adapt to a range of substrates. Unfortunately, the photons emitted from these TMD monolayers are difficult to efficiently collect into simple optics, reducing the practicality of these materials. The realization of on-chip optical devices for quantum information applications requires structures that maximize optical extraction efficiently whilst also minimizing substrate loss. In this work we propose a photonic crystal cavity based on silicon rods that allows maximal spatial and spectral coupling between TMD monolayers and the cavity mode. Finite difference time domain (FDTD) simulations revealed that TMDs coupled to this type of cavity have highly directional emission towards the collection optics, as well as up to 400% enhancement in luminescence intensity, compared to monolayers on flat substrates. We consider realistic fabrication tolerances and discuss the extent of the achievable spatial alignment with the cavity mode field maxima

Increasing quantum light extraction from TMDC's

Much of the recent explosion of research into 2D semiconductor materials has focused on direct bandgap materials such as monolayers of transition metal dichalcogenides (TMDCs), which show great promise in optoelectronic devices such as ultra-thin LEDs [1, 2]. Extraction of light out of these structures can be enhanced in the near field through the integration of these monolayers into waveguides, cavities, or photonic crystals [3]; however these methods are not ideal as they require costly and time consuming processing. Furthermore none of these methods allow you to observe the light directly, therefore are unhelpful in certain applications, such as quantum unique devices [4]. The research we present demonstrates a solution to this problem by encapsulating a range of two-dimensional materials in Solid Immersion Lenses (SILs), dynamically-shaped from UV cure epoxy. We show that the advantages of using SILs formed in this way are numerous, with the most prominent being they can be deterministically placed and directly tuned, to ensure the extraction efficiency is maximised. We will also present detailed photoluminescence maps showing how the reduction of laser spot size caused by focusing through a SIL can allow for very detailed mapping of WSe2 multilayer structures

Curvature-enhanced localised emission from dark states in wrinkled monolayer WSe2 at room temperature

Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. Here we propose an alternative paradigm based on curvature, rather than in-plane stretching, achieved through free-standing wrinkles of monolayer tungsten diselenide (WSe2). We probe these nanostructures using tip-enhanced optical spectroscopy to reveal the spatial localisation of out-of-plane polarised emission from the WSe2 wrinkles. Based on the photoluminescence and Raman scattering signatures resolved with nanoscale spatial resolution, we propose the existence of a manifold of spin-forbidden excitonic states that are activated by the local curvature of the WSe2. We are able to access these dark states through the out-of-plane polarised surface plasmon polariton resulting in enhanced strongly localised emission at room temperature, which is of potential interest for quantum technologies and photonic devices

Increasing the light extraction and longevity of TMDC monolayers using liquid formed micro-lenses

The recent discovery of semiconducting two-dimensional materials is predicted to lead to the introduction of a series of revolutionary optoelectronic components that are just a few atoms thick. Key remaining challenges for producing practical devices from these materials lie in improving the coupling of light into and out of single atomic layers, and in making these layers robust to the influence of their surrounding environment. We present a solution to tackle both of these problems simultaneously, by deterministically placing an epoxy based micro-lens directly onto the materials’ surface. We show that this approach enhances the photoluminescence of tungsten diselenide (WSe2) monolayers by up to 300%, and nearly doubles the imaging resolution of the system. Furthermore, this solution fully encapsulates the monolayer, preventing it from physical damage and degradation in air. The optical solution we have developed could become a key enabling technology for the mass production of ultra-thin optical devices, such as quantum light emitting diodes

Shielding noises from spins

Electrons in InAs/GaAs quantum dots are strong candidates for qubits due to quantum confinement of the spin ½ system. However, electrons couple with nearby nuclear spins and the fluctuating electrostatic environment, these impose an undesired bottleneck on the performance of a quantum spin device. We show that the fluctuating charge and spin environment may be circumvented to an extent. Compared to InAs/GaAs quantum dots, two-dimensional materials may be used to minimise nuclear spin noise; combined with careful sample design a low-decoherence platform is envisioned

Optical identification using imperfections in 2D materials

The ability to uniquely identify an object or device is important for authentication [1]. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Imperfections created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle- adjustable transmission filter, simple optics and a CCD camera can capture spatially- dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting

Pillar-based photonic crystals for light extraction from 2D materials

We present results from pillar-based photonic crystal cavities that have been designed to enhance quantum light extraction from 2D materials. In this work, we calculate the confined field distribution within the cavity and propose exploiting the sagging of 2D materials within the cavity. This has the potential to efficiently couple quantum dots in 2D materials to the maximum intensity of the cavity’s mode. In addition, we propose the use of solid immersion lenses to enhance the vertical confinement of the cavity mode, therefore achieving higher Q-factor whilst remaining in the weak coupling regime. In recent years, 2D materials have created a plethora of new scientific and technological breakthroughs across many disciplines, one being their use in quantum information. Naturally occurring or artificially created defects in 2D materials, such as hexagonal Boron Nitride monolayers, have shown room temperature quantum light emission due to zero-dimensional exciton confinement [1]. This opens the door to a wide range of applications relating to quantum communication [2] and photonic quantum computing [3]. However, it remains difficult to efficiently couple light emitted from 2D materials into optoelectronic systems. In this work, we propose a novel design of a pillar-based photonic crystal cavity for this purpose, as illustrated in the figure 1 (left). 3D finite difference time domain simulations of the structure show that the cavity exhibits an air photonic mode, with an Ex and Ey (figure 1, middle and right¬¬¬ respectively)field confinement with Q-factors exceeding 6000. The 2D supercell contains dielectric pillars in a triangular lattice with a cross-sectional radius 0.185a, where a is the lattice constant. The pillars are surrounded by air in the x-y direction and a silicon wafer surface at the bottom. Above the cavity, we are also proposing the use of a solid immersion lens, to help increasing vertical confinement whilst enhancing light extraction in the vertical direction. The frequency of the confined mode was found to be approximately 0.877a. This method of efficiently coupling of quantum light from 2D materials represents the first step in the design of robust quantum optoelectronic components based on artificial defects in the emerging 2D material system [1] T. Tran et al., Nature Nanotechnology, 242, 37-41 (2015) [2] N Gisin et al., Reviews of Modern Physics, 74, 145 (2002) [3] E Knill et al., Nature, 409, 46-52 (2001

Electrochemical Properties of Metallic Coatings

The metallic coating is an outstanding corrosion-protection option with extensive applications, especially in high-temperature environments. Considering the close relationship between anti-corrosion ability and constitutions, it is necessary to acquire the electrochemical properties of metallic coatings for optimizing their corrosion resistance, and further provide guidance for coating design based on the protection mechanism. Thus, this Special Issue aims at collecting research articles focusing on the electrochemical properties of various metallic coatings, especially on the application of new electrochemical techniques for analyzing the corrosion protection process and mechanism of these coatings. Both experimental and theoretical types of research are welcome for the contribution